I don't know much about keloids, but I think you may want to ask yourself if a lip piercing is really worth the risk. Somehow I imagine your face, and what you currently have, is a lot more important than risking infection, surgery and scarring. Consider focusing on other things. I bet as time goes by having a lip piercing won't be very important anymore. Just go ask an adult that has had a tatoo removed?! :)
If you are prone to keloid scarring there is always a good chance that this will happen at any 'injured' site, especially if infection sets in at the place of the piercing. It's a choice/chance you will need to make. If you go ahead make sure you go to a reputable piercer and be vigilant with post piercing hygiene but being on the mouth you need to be extra careful as your mouth harbors all sorts of germs and you also put food etc in there as well.
Keep it very clean! Keep dirty hands away from it, that means don't touch it ever without washing first and don't lick it either - hygiene, hygiene, hygiene! Wash your hands thoroughly with warm soapy water and dry them on a clean towel. If you see any signs of infection get antibiotics asap. The longer infection is there the more the hole will get bigger by the infection killing the cells/skin around the hole causing scarring. The body piercer should give you a specific bacterial wash for you to use after getting pierced (they do here in Australia). Otherwise you'll need to buy an antiseptic wash liquid to use yourself. Good Luck. ;)